1. Field of the Invention
The present invention relates to a nondestructive testing apparatus, and more particularly to a nondestructive testing apparatus capable of testing quality of a large-sized structural concrete partition buried in the seabed or in the earth, for example, a large-diameter placing pile, and Caisson line, etc.
2. Description of the Related Art
With increasing industrialization, fundamental components indicative of large-sized structures, such as bridges and harbors, have been rapidly increased. The above-mentioned bridge or harbor structure may collapse if unexpected holes occur during a curing period for a concrete pile contained in the structure, such that the necessity of estimating if the structure is in a normal state has been rapidly increased.
There have been widely used first to third methods to estimate if the structure is in a normal state. In the case of the first method, a user excavates peripheral parts of a concrete pile, and inspects the presence or absence of damage of the concrete pile and the position of the damage. In the case of the second method, a user strikes the concrete pile with a hammer, detects elastic waves using a specific sensor, and inspects the presence or absence of damage of the concrete pile and the position of the damage on the basis of the shape of the elastic waves. In the case of the third method, a user performs core boring of the concrete pile, measures the concrete pile and its peripheral displacement using a clinometer, and inspects the presence or absence of damage of the concrete pile and the position of the damage.
However, the first method, in which the user performs excavation and views the damage of the concrete pile, requests a large-sized excavating operation, such that it requires large-scale equipment, and consumes an unnecessary long period of time and great cost. The second method using the elastic waves has poor precision, such that it must be used along with the above-mentioned first method at the same time. The third method using the clinometer requires boring, such that it consumes an unnecessary long period of time and great cost.
In order to above-mentioned problems, a nondestructive testing apparatus shown in FIGS. 1a˜1b is developed.
FIGS. 1a˜1b show block diagrams illustrating a conventional nondestructive testing apparatus. FIG. 1c shows a conventional method for detecting a defect using ultrasound. FIG. 1d shows an example for illustrating a reception system problem encountered when the conventional nondestructive testing apparatus is used.
The conventional nondestructive testing apparatus shown in FIG. 1a includes a transmitter 10 for generating one-way ultrasound; a receiver 20 for receiving the ultrasound from the transmitter 10; a microprocessor 30 for generating a control signal to control the transmitter 10 to generate the ultrasound, receiving ultrasound information from the receiver 20, analyzing the received ultrasound information, and determining the presence or absence of internal quality abnormality of the concrete pile; and an output unit 40 for printing out the determined result of the microprocessor 30 or displaying the same determined result on a screen.
In this case, the nondestructive testing apparatus shown in FIG. 1a connects the transmitter 10 to the receiver 20 on a one to one basis, such that it requires a plurality of transmission and reception processes to perform overall quality testing. Also, the nondestructive testing apparatus entirely depends on a manual operation of a user who carries out the nondestructive testing, such that reliability of the quality testing result is deteriorated if the user is unskilled at the quality testing or makes an unexpected mistake. Also, the user must hold the transmitter 10 and the receiver 20, or must attach the same to a necessary part, such that it has difficulty in performing the above-mentioned testing on inaccessible areas such as the seabed and the subterranean parts.
In order to solve the above-mentioned problem, a nondestructive testing apparatus shown in FIG. 1b has been developed. The nondestructive testing apparatus shown in FIG. 1b includes a transmitter 11 inserted in a vertical hole to generate ultrasound in a radial direction of 360°; a plurality of receivers 21, 22, and 23 inserted in vertical holes different from the vertical hole of the transmitter 11 at the same depth to receive the ultrasound from a concrete partition; a measurement depth controller 500 for allowing the transmitter 11 and the receivers 21˜23 to maintain the same depth; and a microprocessor 31 for analyzing speed of the ultrasound received from the receivers 21˜23 according to an internal program, determining density and quality of concrete, and displaying the determined density and quality of the concrete.
In this case, although the nondestructive testing apparatus shown in FIG. 1b is able to perform an overall quality test of the structure using only one transmission/reception process, it uses a low frequency of about 50 kHz as a transmission frequency, such that it is unable to recognize a defect of the structure. The conventional nondestructive testing apparatus shown in FIGS. 1a˜1b may encounter the following first to third problems.
According to the first problem, the conventional nondestructive testing apparatus cannot detect small-sized defects of less than a predetermined size. A resonance frequency 50 kHz of a transmission probe (i.e., a transmitter) of each conventional nondestructive testing apparatus based on the conventional ITS-2002 specification controls its own waveform to have a wavelength of 8 cm when speed of ultrasound passing through the concrete is 4,000 m/sec. Generally, the size of each internal defect contained in the concrete is bigger than the 8 cm, such that there is no need to detect a small-sized defect less than the 8 cm. As such, nondestructive testing apparatuses employing the above-mentioned frequency band have rapidly come into widespread use. However, as the number of large-sized structures is rapidly increased, small aggregate of less than 5 cm is used as specific aggregate (e.g., gravel or crushed aggregate) capable of maintaining high strength of the structure, and the use of the small aggregate less than 5 cm is commercially available. It is impossible for the transmission/reception probe for use in the conventional concrete nondestructive testing apparatus to detect an internal defect (e.g., a cold joint) of the concrete of less than 5 cm. In other words, in order to detect a small-sized concrete internal defect such as a cold joint considered to be an important matter in a construction site, a frequency range of a transmission/reception probe must be extended to 50 kHz˜400 kHz and over, but it should be noted that a current transmission/reception probe has yet to overcome the above-mentioned limitation.
According to the second problem, the conventional nondestructive testing apparatus cannot recognize a defect in three dimensions. In other words, a transmission probe of the conventional nondestructive testing apparatus receives ultrasound, and a reception probe thereof receives ultrasound having passed through an internal medium of the concrete. Therefore, a user compares an initial ultrasound arrival time in a normal concrete medium with the other initial ultrasound arrival time in a defective medium, determines the size of the defect, and reports the defect to the person in charge, such that the user can predict type and shape information of the defect. However, the above method requires analysis of th upper/lower homogeneity of the concrete using the logging of the transmission/reception probes, and requires analysis of data of a reception point at different vertical distance transmission locations using the same logging, such that it has difficulty in measuring position and size information of a concrete defect oriented vertically in a large-diameter pile. In order to measure the position and size of the defect contained in the concrete, a plurality of measurement holes must be buried before constructing the large-diameter pile, resulting in consumption of an unnecessarily long period of time and great cost. The conventional nondestructive testing apparatus analyzes the data obtained by the above-mentioned process, and predicts only the size of the defect contained in the concrete, such that it is unable to measure at least two-dimensional result.
According to the third problem, the conventional nondestructive testing apparatus is configured in the form of an analog signal reception system as shown in FIG. 1d, such that its inspection function is limited. In other words, a transmission probe for use in the conventional nondestructive testing apparatus generates a resonance frequency of 50 kHz, and uses a long wavelength due to the resonance frequency of 50 kHz, such that the conventional nondestructive testing apparatus is unable to predict a small defect contained in the concrete whereas it has a very long ultrasound measurement distance. Also, the conventional nondestructive testing apparatus has a limitation in performing a gain control operation. For example, provided that speed of ultrasound passing through the concrete is 4,000 m/sec, an initial ultrasound arrival time is shown in the following Table 1.
TABLE 1Ultrasound Time During Passing 1M250 μsUltrasound Time During Passing 2M500 μsUltrasound Time During Passing 3M750 μs
In this case, a signal display unit for use in a conventional concrete nondestructive testing apparatus using an analog signal controls an ultrasound frequency to be adjacent to a predetermined frequency of 50 kHz, each of all time divisions for use in the nondestructive testing apparatus is limited to 200 μs, and an overall time division associated with a horizontal axis of an analog reception signal is divided into five stages, such that a maximum width may be denoted by 200 μs×5=1000 μs. As a result, the initial ultrasound arrival time denoted by the waveform shown in FIG. 1d is generated in almost the last part of the waveform, such that the conventional nondestructive testing apparatus has difficulty in analyzing reception data.
In conclusion, the above-mentioned conventional analog-type concrete nondestructive testing apparatus widely used to inspect large-diameter piles fixes an overall time division to 1000 μs, such that an initial waveform is slightly generated in the last part of a display when testing defective concrete. As a result, the above-mentioned nondestructive testing apparatus is unable to measure a horizontal length of the defect and energy variation. In addition, if the width of the concrete structure is increased, the size of the time division is higher than 1000 μs, such that the nondestructive testing apparatus is unable to detect concrete defects.